1. Field of the Invention
This invention relates to vacuum enclosures and more particularly to spacers for location between two sheets defining opposite walls of a vacuum enclosure.
A pair of spaced rigid sheets of glass or similar transparent material is used in a number of applications to form opposite walls of a vacuum enclosure. Examples of such applications are double glazing units and solar panels in which a solar plate is located within the evacuated space between the sheets.
Where high degrees of vacuum are employed, spacers are necessary between the sheets in order to maintain the sheets at the correct spacing against the action of atmospheric pressure.
2. Description of the Prior Art
Particularly in the application to double glazing units or to solar panels, where high vacuums are employed, it has been found that in operation, relative movement between the sheets occurs as a result of expansion and contraction caused by differences in temperature of the sheets. The spacers between the sheets must therefore be designed to accommodate this relative movement. This ability to take up relative movement of the sheets also applies to the design of the vacuum seals around the edges of the enclosure, which, in addition to allowing this relative movement to take place, must at the same time maintain an effective vacuum seal.
It has been proposed to use rocking spacer pins, which are designed to rock following relative movement of the sheets. These spacer pins comprise a cylindrical shank formed at one end with a spherical bearing surface. Each end is received in the central recess of a counterbored washer which acts as a bearing plate to spread the load at the glass surfaces. The spacer pin pivots about the cylindrical end, and the spherical bearing surface at the other end is designed so that the pin has an equilibrium position at right angles to the glass surface. When the pin is rocked to either side of the equilibrium position, the curvature of the bearing surface is designed to move the glass sheets slightly apart, so that the force exerted by atmospheric pressure tends to return the pin to its equilibrium position.
It has been found that this simple design of spacer has certain problems. The spacers have to be made extremely accurately to ensure that they are all of equal length. If they are of unequal length, the glass is not supported uniformly and suffers breakages. Thus the length of each spacer pin and the thickness of each washer has to be made extremely accurately, or they have to be selected to ensure that the overall length of each spacer is equal. Also in continual use, the cylindrical end of the spacer can get worn and distorted, and tends to gouge out the surface of the counterbored washer, and this wear changes the effective overall length with time.
The present invention proposes a spacer assembly which provides an effective separation of the sheets whilst at the same time allowing relative movement thereof, and which is capable of accommodating small variations and tolerances in spacer length and in the thickness of the material of the sheets. The invention also provides a vacuum enclosure which incorporates these spacers, and which is provided with a simple but effective design of edge seal.